With the increase in heat dissipation from microelectronic devices and the reduction in overall form factors, thermal management becomes a more and more important element of electronic product design. Both the performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment.
Heat sinks function by efficiently dissipating thermal energy (i.e., “heat”) generated from an object (e.g., electronic module or microelectronic component) into a cooler ambient, e.g., the air; and at least transfer thermal energy from an object at a high temperature to a second object at a lower temperature with a much greater heat capacity.
In a common design of a heat sink, a metal device having a flat surface (e.g., a copper or aluminum base) is provided with an array of combs or fin-like protrusions to increase the heat sink's surface area contacting the air and thus increase the heat dissipation rate. A high thermal conductivity of the metal base combined with its large surface area provided by the protrusions result in the rapid transfer of thermal energy to the surrounding, cooler air.
Liquid cold plates, on the other hand, provide an alternative advantage over some air cooled solutions in high watt density applications and may include tubed cold plates, flat tube cold plates, performance-fin cold plates and liquid-cooled chassis designs.
Flexible cold plates have been proposed as a means of reducing the cost of cooling multiple neighboring devices
U.S. Pat. No. 8,736,048 addresses flexibility improvements of heat sink and cold plate designs with “lateral compliance features”. For example, FIG. 1A shows conceptually a prior art heat sink design that incorporates lateral compliance into a vertically flexible/compliant heat sink which allows strain to be absorbed by the heat sink rather than by the thermal interface material (TIM) that provides an interface for heat exchange between the chip and the heat sink. As shown in FIG. 1A, the vertically flexible/compliant heat sink 10 with lateral compliance is shown secured in heat exchange relation with electronic components, e.g., chips 12, mounted on a carrier or substrate 11 of an electronic chip module (e.g., a multi-chip module or MCM). In this design, each chip 12 terminates in chip contact patches 13. The heat sink 10 includes a thin thermally conductive flexible heat sink sheet 14 having lateral compliance that extends in a plane over the chips 12 and is secured to chips 12 through chip contact patches 13 utilizing a thermal interface material (TIM) (not shown). Heat transfer elements (HTE) 16 such as fins, pins or other heat transfer structures are mounted on flexible heat sink sheet 14 having lateral compliance and support a thin thermally conductive flexible heat sink sheet 15 having lateral compliance that also extends in a plane above the MCM device. Flexible heat sink sheet 14 having lateral compliance HTE 16, flexible heat sink sheet 15 having lateral compliance, define channels 18 through which a cooling fluid or heat exchange fluid flows to carry away any heat developed in the MCM.
The thin top and bottom heat sink sheets 15 and 14 provide flexibility to accommodate tilted chips and level differences between chips. Adding lateral compliance features, i.e., bends 17, to at least the bottom (chipside) sheet 14, and optionally sheet 15, allows for lateral expansion and contraction of the heat sink relative to the substrate without moving or significantly stressing the individual chip contact patches 13. That is, the heat sink 10 of FIG. 1A provides a lateral compliance feature that minimizes or eliminates shear stress and shear strain developed in the horizontal direction at the interface between the heat sink and the ship contact patches by allowing for horizontal expansion and contraction of the heat sink relative to the underlying electronic chip module without moving the individual chip contact patches in a horizontal direction.
In a further prior art heat sink 20 shown in FIG. 1B, the thin top and bottom heat sink sheets 15 and 14 provide flexibility to accommodate tilted chips and level differences between chips by adding a lateral compliance feature in the form of an elastomer 21 that secures the facing, substantially parallel edges of adjoining separate heat sink sheets to allow for lateral expansion and contraction of the heat sink relative to the substrate without moving or significantly stressing the individual chip contact patches.
These lateral compliance features are more useful when the active area can be distanced from the sidewall and when the height difference between elements to be cooled is relatively small. When the height difference increases and the active area is near the allowed width of the flexible heat sink, such features are less effective. Such features also complicate flow control/blocking outside the active area.
In certain systems, the load that can be applied to force active areas (e.g., areas where heat is primarily transferred) of the flexible cold plate in to contact with the element being cooled is constrained. In addition, the load applied to the heat sink can produce non-uniform loading of the element being cooled if the heat sink is too stiff. For adequate thermal performance, a cold plate has a minimum height in the active area, and the system constraints can limit the overall width of the cold plate. These constraints can result in an overly stiff flexible cold plate.